1. Field of the Invention
The invention concerns a towing vehicle for aircraft, with at least one operator""s cabin, which includes control elements for steering, accelerating and braking the towing vehicle, in addition to a coupling member for creating a detachable connection between the aircraft and the towing vehicle, wherein the towing vehicle can be braked by hydraulic or pneumatic brakes acting on at least two wheels of the vehicle and wherein the braking control element is configured as a brake pedal which actuates the brakes either directly or via a control circuit.
Aircraft towing vehicles are motorized vehicles which must tow, push and brake a substantially greater mass than their own mass. Accordingly there are special requirements in particular for the braking system of the towing vehicle, in particular when taking into consideration that the operational safety of the towing vehicle must be maintained under very diverse operating conditions (either towing or operating without load). Aircraft tow vehicles can in general be divided into xe2x80x9ctowbar-tractorsxe2x80x9d and xe2x80x9ctowbarless-tractorsxe2x80x9d. Although the present invention was particularly desired for towbar tractors, it can however be utilized also for towbarless tractors.
2. Description of the Related Art
Known aircraft towing vehicles are provided with a conventional hydraulic brake, wherein the brake pedal acts directly on a brake medium (hydraulic fluid). The brake force is therein directed through proportional valves, which produce a braking effect proportional to the brake pedal position. Also known are servo-controlled brakes, wherein the brake pedal opens or closes a control circuit, which controls a release valve, which activates the braking effect in the main circuit. One disadvantage of the known braking systems is comprised therein, that between the brake pedal or as the case maybe the main brake valve provided in immediate proximity to the brake pedal and the brakes, a relatively long brake line must be provided. Besides the high cost of running long circuits there is the substantial disadvantage of the known brake systems that on the basis of the elasticity of the hydraulic lines (stretching or expansion of brake hoses under pressure) delays occur between the actuation of the brake pedal and the initiation of the braking effect, which results in an imprecise brake operation. In the case of so-called spongy brakes there can occur over-braking or under-braking reactions, and besides this feedback occurs on the pedal, which feedback is felt by the operator. In general, the known brake system has little sensitivity.
A further disadvantage of the known brake system is comprised therein, that the hydraulic lines must be continued up to the operator cabin. Heat and noise produced in the brake system is transmitted into the operator cabin in a distracting manner. Besides this, there is the danger of an accident due to rupturing of brake hoses or due to leakage in the operator area. Finally, it has been found to be disadvantageous in the known brake systems, that supplemental signals from operating sensors can only be indirectly introduced into the system. Dynamic contributory effects can practically not be compensated for due to the inertness or response lag in the hydraulic system.
Beginning therewith it is the task of the present invention to provide a brake system for a tow vehicle for aircraft, which is simple in construction, which exhibits a high sensitivity during braking and provides for small response delays by using short hydraulic paths and which can be modified to take into consideration supplemental control variables or, as the case may be, process variables, which result from the varying operating conditions.
The invention is based on the recognition that by employing an electronic brake control the otherwise necessary hydraulic complexity can be minimized, and that in such a system utilization can be made of various supplemental signals, provided by operating sensors, for controlling the brakes. According to the invention it is thus provided that the brake pedal is coupled with a position or angle sensor, which translates the pedal position or angle into an electrical desired value position signal, which controls via an electrical signal transmission circuit at least one electro-hydraulic brake valve. Preferably, for safety reasons, two separate brake circuits are provided, which are respectively controlled together by operation of the brake pedal. Herein each brake circuit preferably includes a brake pressure controlling main brake valve configured as a proportional valve and including an actuating ram, which controls either the flow-through cross section or, preferably, the pressure in the brake circuit.
It is important during towing as well as during pushing of an aircraft that the operator of the tow vehicle is in control in the respective directions of travel of the vehicle. Since in general during the maneuvering of an aircraft both towing as well as pushing is involved, in a preferred embodiment of the invention two operator cabins are preferably provided at the respective ends of the vehicle, which are respectively provided with the identical control elements.
The main brake valve can be operated by, respectively, an electronically controlled adjustment piston actuatable by a magnet valve and including a displacement pick up, wherein the displacement pick up provides an electronic actual position value for the control circuit. For safety reasons the adjustment piston should include a ram separated from the operating ram and in opposition thereto.
In general it is also possible, in place of the hydraulic brake, to also employ a cable control brake or a pneumatic brake, which can be controlled in comparable manner. It is significant that in every case no additional measuring points are required in the brake medium, which would be necessary for controlling or regulating the system. The actual control of the brake system occurs in the preceding stage prior to the brake actuating hydraulic circuit.
In a preferred embodiment of the invention the control circuit includes an electronic computer as control device for the brakes, which receives at least the desired position signal component of the brake signal as a control value. The control circuit accordingly includes the pedal as desired value provider, while the actual value is detected at the displacement pick up of the adjustment position for the main brake valve. By means of the electronic computer the actual value is continuously compared with the desired value and, when necessary, appropriately adjusted.
The coupling means between aircraft and tow vehicle is preferably configured as a tow rod, wherein a coupling element for the tow rod is to be provided at least one of the vehicle end areas. Preferably the coupling element includes a sensor, preferably designed as an angle provider, for the tow, angle, as well as a sensor, preferably formed is a tension measuring strip, for determining the towing and/or pushing forces between tow vehicle and aircraft. Further, a sensor can be provided at the tow vehicle for the vehicle acceleration as well as the steering position. At these sensors electrical signals can be extracted as parameters for the control of the brakes and/or drive forces of the tow vehicle, which can be supplied to the control device as input signals. The control device preferably includes an electronic storage unit in which characteristic curves are stored for the brakes and/or propulsion control depending upon the values of the signals detected at the sensors, wherein various characteristic curves can be stored in the electronic memory unit for the various characteristic curves for the tow operation for the various aircraft types as well as various characteristic curves for the towing operation vs. for no-load operation. For no-load operation there is produced with the same brake pedal operation only a fraction of the brake-force produced during a towing operation.
Using the values detected by the various sensors, a comprehensive or adaptive control during braking of the tow vehicle can be achieved both during no load operation and during towing operations. By taking into consideration for the brake control those pushing or towing forces exercised on the towing vehicle by the towed aircraft, an adaptation of the brakes of the tow vehicle to the above-mentioned forces is possible. If additionally the towbar angle is taken into consideration as a control value, instabilities in the towing connection can be effectively prevented, which could occur when this angle exceeds the range of tolerance by even a few degrees. In such a case the brake force can automatically be reduced as the position of the brake pedal remains the same, in order to prevent a breaking away of the towed aircraft. A situation of this type occurs particularly when braking must occur while towing in a curve or corner. In certain cases, for prevention of critical situations, the brake force can even be reduced to zero and the drive motor of the towing vehicle can only be acted on for acceleration. Further, steering (wheel) position can also be taken into consideration as a control parameter.
It is further possible to control the brake force depending upon the vehicle speed, or in for example to established a stronger braking effect at higher towing speeds, so long as this is permissible taking into consideration the other control parameters. The simultaneous evaluation of the mentioned control parameters makes possible an optimal braking even under difficult conditions.
A further improvement of the brake control is achieved thereby, that for various predetermined aircraft types various brake characteristic curves are stored in the memory unit of the control device depending upon factors such as the weight and other relevant values of the aircraft types.